A cataract is a clouding that develops in the crystalline lens of the eye or in its envelope (lens capsule), varying in degree from slight to complete opacity and obstructing the passage of light. Cataracts may be partial or complete, stationary or progressive, or hard or soft. Generally, as cataracts progress, the hardness or toughness of the cataract increases. Cataracts are sometimes treated by cutting the affected lens using a scalpel and removing the lens from the eye before replacing the lens. Such treatment can require large incisions to the eye and can put sensitive, non-lens tissue at risk, especially as a cataract hardens and becomes more difficult to cut.